Spark-ignited internal combustion engine oxide gas absorbing arrangement and method

ABSTRACT

In the embodiments described in the specification, a spark-ignited internal combustion engine has an exhaust line containing an oxide gas absorber for absorbing NO x  and SO x  and desorbing NO x  and SO x  at elevated temperatures. The support member for the oxide gas absorber is a metal foil or a thin walled ceramic support to permit rapid heating and is coated with a layer of a gas absorbing material at least 50 microns thick to permit longer intervals between regeneration, thereby providing effective storage of oxide gases even with fuel consumption-optimized engines.

REFERENCE TO RELATED APPLICATION

This is a divisional of copending application Ser. No. 09/252,507 filedFeb. 18, 1999, which is a continuation of International Application No.PCT/EP97/04307, filed Aug. 7, 1997.

BACKGROUND OF INVENTION

This invention relates to absorbing arrangements and methods fortemporarily storing oxides of nitrogen and sulfur and removing suchoxides from the exhaust gas from spark-ignited internal combustionengines.

As used herein, the term “absorb” includes the chemical process forstoring gases such as, for example, by conversion of barium oxide tobarium nitrate for storage of nitrogen oxide.

U.S. Pat. No. 4,755,499 discloses an arrangement for the reversiblestorage of oxides of nitrogen and sulfur, for example from motor vehicleexhaust gases, in which the absorber is regenerated by heating in areducing atmosphere. In this arrangement, a reduction of the nitrogenoxides takes place at the same time.

A storage catalyst of that type for use in motor vehicles is describedin more detail in U.S. Pat. No. 5,402,641, in which high temperaturesabove 500° C. are necessary to regenerate the absorber. Consequently,use of the storage catalyst is possible only for motor vehicles having ahigh exhaust gas temperature, in particular for motor vehicles with anOtto engine.

In this case, however, the possibility of use is limited since, undercertain operating conditions of internal combustion engines, such asoccur for example in city traffic, the acceleration phases cause a largeemission of nitrogen oxide, but no long lasting high temperaturecondition such as is required to regenerate the absorber, especiallywith respect to oxides of sulfur, is attained.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide aspark-ignited internal combustion engine arrangement and method forreleasably absorbing oxides of sulfur and nitrogen in exhaust gaseswhich overcomes disadvantages of the prior art.

Another object of the invention is to provide a spark-ignited internalcombustion engine having an absorber for nitrogen oxides in exhaustgases and a corresponding method, suitable especially for use with fuelconsumption-optimized engines such as direct injection Otto engines, inwhich regeneration of the absorber is possible even at low exhaust gastemperatures.

These and other objects of the invention are attained by providing aspark-ignited internal combustion engine arrangement having an absorberfor absorbing oxide gases which includes an absorption layer forabsorbing oxides of nitrogen and/or sulfur on a support member and acontrol unit for controlling the temperature of the absorption layer sothat the layer can be heated to a temperature at which it is regeneratedby desorbing the NO_(x) and/or SO_(x) even at very low exhaust gastemperatures such as occur for example in the case of direct injectionengines.

According to the invention, the usual gas absorbing materials may beemployed, for example as described in U.S. Pat. No. 4,755,499, and alsoin U.S. Pat. Nos. 5,402,641 and 5,362,463. A common feature of all thesestorage materials is that they have an elevated absorption temperature,while a still higher regeneration temperature is required especially forremoving the oxides of sulfur. For most storage media of this kind,temperatures in the range from 150° to 700° C., in particulartemperatures above 300° C., are required. Such temperatures commonlyoccur in motor vehicles with Otto engines, but are comparatively rarewith Diesel engines and especially in internal combustion engines havingdirect fuel injection.

The preferred NO_(x) storage materials are distinguished in that, underconditions of net oxidation, i.e., a stoichiometric excess of oxidizingagents, such as occurs in the exhaust gas during the operation, theywill store nitrogen oxides and, upon a reduction of the excess ofoxygen, may reduce them. For this purpose, the NO_(x) storage catalystsusually include a precious metal, in particular the usual precious metalcoatings for three-way catalysts. The NO_(x)-laden storage material isadvantageously regenerated in a regenerating phase at λ≦1.

Ordinarily, various reactions take place successively or simultaneouslyon the NO_(x) storage catalyst, the most important reactions being:oxidation of the NO in the exhaust gas to NO₂, storage of the NO₂ asnitrate, decomposition of the nitrate, and reduction of the re-formedNO₂ to nitrogen and oxygen.

As described above, the course of the reactions depends, among otherthings, not only on the temperature of the catalyst but also on theconcentration of the reagents at the active region of the catalyst andthe flow velocity of the gas.

According to the invention, it has now been found that, with variousfactors capable of being combined with each other, it is possible also,at little cost, to optimize the known exhaust gas absorbers so that theymay be employed for spark-ignited internal combustion engines withdirect injection. For this purpose, the wall thickness of the supportingmember on which the absorption layer is applied preferably should be≦160microns, and desirably≦140 microns and if a metal support is used, awall thickness≦50 microns, preferably≦40 microns, and desirably≦30microns, and the absorber should preferably be heated to a temperatureabove the temperature of the exhaust gas.

According to the invention, it has been found that, with the use of thinwalled ceramic supports for the absorption layer, i.e. supportingmembers having a wall thickness≦0.14 mm, not only is a more rapidtemperature rise of the absorption layer possible, but also a thickerabsorption layer may be used. This accomplishes two objectives: in thefirst place, even short periods of high temperature operation can beutilized for regeneration since the storage layer temperature will beincreased to the required temperature more quickly, and in the secondplace, by providing a thicker absorption layer, a higher oxide gasstorage capacity can be achieved so that during operation of theinternal combustion engine a longer period of time can elapse before thestorage layer must be regenerated. Consequently, despite the lessfrequent occurrence of temperature peaks in the exhaust gas ofconsumption-optimized internal combustion engines, no failure of thestorage layer resulting from exceeding its storage saturation limit willoccur.

According to the invention, absorbers having a support member made ofmetal foil are especially suitable, and the metal foil mayadvantageously be connectable to an electric power source for resistanceheating so that, even at low exhaust gas temperatures, the absorber canbe brought to the necessary regenerating temperature by passing anelectric current through the metal support. Furthermore, by using ametal support member, the gas passages which are coated with theabsorption layer may be variously shaped, so that, for example, acontrolled turbulent flow vortex of the exhaust gas in the passages canbe established.

With especial advantage, according to the invention, supports with avariety of passage segments may be used for the absorber where, forexample, an intermediate region of the passages is modified to produce aturbulent flow. This can be done, for example, by varying the passagecross-section, or by a twisting or distortion of the passages. In thisway, the support may be adapted in a controlled way for especiallyfavorable reaction conditions along the flow passages. Another specialfeature of the support, beside a possible variation in number ofpassages in the flow direction and the provision of changes of crosssection along the flow direction, is the segmentation of the supportwhere, for example, one segment with an absorption layer is disposednear the engine outlet and another segment with an absorption layer islocated somewhat farther away. Thus, even with the most variableoperating conditions, good NO_(x) purification results can be obtainedwith fuel consumption-optimized engines.

According to the invention, it has been found that the oxide gas storagearrangement will have especially good absorption and desorptionproperties if the flow passages for the exhaust gas are distorted in anintermediate region to achieve a turbulent flow between an inlet regionand an outlet region which do not have a distorted structure to produceturbulent flow. As a simple arrangement for generating such a turbulentflow, for example, a transition from a large to a small diameter in thepassages is effective, but twisting of the entire support in anintermediate region will also serve to generate turbulence. Theespecially favorable properties resulting from such turbulent flow arepresumably achieved by a division of the individual reaction stepsrequired to reduce nitrogen oxides among the successive regions of thesupport, with a modified intermediate region affording better conditionsfor reaction than unmodified intermediate regions.

To produce especially good oxide gas conversions, the absorption layerhas an enlarged surface area, that is, a total surface area that issubstantially larger than the area of the surface of the support memberon which it is coated. For this purpose, the absorption layer provides asurface area to which the exhaust gas is exposed of at least 20 m²/g,and preferably at least 40 m²/g. Also, the absorption layer preferablyhas a pore volume of at least 0.2 cm³/g, and desirably at least 0.4cm³/g, a bimodal pore size distribution with both micropores andmacropores also being acceptable. This may be achieved for example bythe choice of the size of the particles forming the absorber surface, inwhich mixtures or specified distributions of different particle sizesare also suitable.

An especially suitable absorption material is gamma-aluminum oxidecontaining one or more elements in the group consisting of alkalimetals, alkaline-earth metals, rare earths and/or lanthanum. Thepresence of the elements copper and manganese is also suitable. Theadded elements are usually present as oxides, or else as carbonates ornitrates, the storage effects being achieved by formation ofcorresponding nitrates and sulfates, which are then converted back tooxides or carbonates under the appropriate reaction conditions. In thisway, it is possible to absorb NO_(x) and/or SO_(x) from an exhaust gascontaining at least 1% oxygen.

As described above, the absorbed substances are desorbed from thestorage catalyst layer by elevated temperatures and in a reducingatmosphere. For this purpose, it is desirable to determine the oxygenconcentration in the exhaust gas so that the oxygen concentration, or aquantity having a known relationship to the oxygen concentration, can beutilized to control the process of absorption or desorption.

Since the temperature of the absorption layer, determined directly orindirectly, is also important the same consideration is also applicableto the temperature of the exhaust gas. Thus, the absorption layertemperature may for example be determined by measuring the temperatureof the exhaust gas or of the support member. A determination oftemperature over the operating diagram of the internal combustion engineis also possible.

With the present invention, absorption layers having a thickness of atleast 50 microns, preferably at least 70 microns, and desirably at least90 microns, can be provided. These values are average layer thickness ofa cross section and should extend over preferably at least 50% anddesirably at least 80% of the total absorber. The foregoing absorptionlayer thickness values apply to layers on ceramic substrates. Half ofthose values apply to absorption layers provided on metal substrates.Such high layer thickness values permit a greater storage capacitycompared to conventional absorbers, and consequently permit longerintervals between regeneration as described above.

According to the method of the invention, regeneration of the absorptionlayer is preferably carried out when the operating conditions of theinternal combustion engine produce a correspondingly high temperature ofthe exhaust gas and hence of the absorption layer. Especiallyadvantageous, however, is a method in which supplementary heating of theabsorption layer is provided, preferably electrically. Other possibleheating procedures include ignition control measures in Otto engines,variation of lambda, lowering of lambda below 1, and addition ofsecondary air to generate exothermia on an oxidation catalyst, and/or anexhaust ignition arrangement, as well as heating the catalyst with aburner. A segmented absorber in which the segments are heated accordingto the required reaction is especially advantageous. Thus, for example,only one absorber segment located in a downstream exhaust flow directionmay be heated, especially in case of a distinct spatial separation ofthe absorber segments. Electric heating is especially advantageous inthis case, but an injection of fuel into the exhaust gas and/or a burnermay also be used. By arranging individual segments for individualreactions at different distances from the engine exhaust manifold,thermal aging of the absorber may be reduced in addition to providingthe advantage of especially favorable reaction temperatures inindividual absorber segments.

Since the release and conversion of the NO_(x) from the storage layerand the release of the oxides of sulfur from the storage layer requiredifferent temperatures, higher in the case of the sulfur oxides, it isalso possible to proceed so that a desorption of the oxides of sulfur,which are present in particular as sulfate is performed at longer timeintervals or only as needed, so that the storage layer is onlyoccasionally heated to the high temperatures needed for desorption ofthe sulfur oxides. This counteracts premature aging of the storagelayer, so that an especially good long term stability of the absorber isachieved. This procedure may be used with the absorber arrangement andmethod described above.

BRIEF DESCRIPTION OF THE DRAWING

Further objects and advantages of the invention will be apparent from areading of the following description in conjunction with theaccompanying single drawing illustration which shows schematically arepresentative embodiment of a spark-ignited internal combustion enginehaving an exhaust gas absorbing arrangement according to the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

In the typical embodiment of the invention shown in the drawing anabsorber 1 of oxide gas components is mounted in an exhaust line 2 of anOtto-type internal combustion engine 3 the operation of which iscontrolled by a motor control unit 4. The motor control unit 4 controlsan injection pump 5 delivering fuel from a tank 6 (not shown) to a fuelinjection nozzle 7. In addition, the motor control unit 4 controlsignition of the fuel-air mixture in each engine cylinder by a spark plug18. For the sake of clarity, the numerous conventional control lines forfuel and air supply and discharge and the like leading to the motorcontrol unit 4 are not shown.

In the illustrated exhaust line 2 a three-way catalyst 8 is mounteddownstream from the absorber 1 but it is also possible to locate thethree-way catalyst 8 upstream from the absorber 1.

In this embodiment, the absorber 1 is made from two metal foils one ofwhich is smooth and the other of which is corrugated and is connected tothe smooth foil by soldering at the corrugation crests. By rolling thismultilayer foil together, a cylindrical member having a plurality ofcoaxial passages is provided. In addition, the support member of theabsorber has an intermediate region 15 which is twisted about itslongitudinal axis, so that the individual passages 16 in theintermediate region are narrowed and contorted to produce turbulence inthe exhaust gases flowing through the passages. A similar turbulencegenerating structure in the intermediate region may also be achieved byproviding transverse corrugations in one or both of the metal foils.

The metal foil material contains a few percent of aluminum and isanodized so that a “wash coat” containing gamma-aluminum oxide willadhere better to the metal foil surface. The aluminum oxide wash coatfurther contains one or more of the elements sodium, barium, cerium andlanthanum, providing a layer on the aluminum oxide containing the salts,i.e., nitrates, oxides and hydroxides of those elements. By impregnatingthe wound support foils with the wash coats and then firing, theabsorbing layer with those salts is produced. Additionally, theabsorbing layer is impregnated with a solution containing salts of theprecious metals platinum and rhodium and possibly palladium in additionto or instead of the rhodium, from which the corresponding preciousmetals are then liberated during firing. The resulting precious metalcoating provides a three-way catalyst. The oxide gas absorber isprovided with an electrical contact 9 and mounted in a housing so thatan electric current can be passed through the foils and the absorbinglayer to a grounded housing. The electrical contact 9 is connected tothe control unit 4, and a temperature sensor 10, also connected to thecontrol unit 4, is mounted inside the housing.

Upstream from the absorber 1, a broad-band lambda probe 11 inserted inthe exhaust gas pipe, provides signals which are proportional to theoxygen concentration present in the exhaust gas to the control unit 4.In addition, a fuel injector 12 is mounted upstream from the lambdaprobe 11 and is supplied with fuel on instructions from the control unit4. Between the absorber 1 and the following catalyst 8, an air injector13 is provided, receiving air from a pump 14 controlled by the controlunit 4.

The internal combustion engine 3 is an Otto-type engine with directinjection, producing exhaust gas which normally has a large excess ofoxygen and a temperature of about 200° to 500° C. In operation of theinternal combustion engine, nitrogen oxides and oxides of sulfur presentin the exhaust are absorbed by the absorption layer of the absorber 1 inthe form of nitrates and sulfates of sodium, barium, lanthanum and thelike, while at the same time any oxidizable constituents present, mostlyhydrocarbons, are oxidized by the precious metal coating of the absorber1.

When the saturation limit of the absorbing layer in the absorber 1 isreached, or else at predetermined time intervals or in response to othercontrol parameters, such as for example a determination of NO_(x) in theexhaust following the absorber, the absorber is regenerated, i.e. freedfrom the NO_(x), incorporated for example as barium nitrate. At the sametime, oxides of sulfur, incorporated for example as barium sulfate, maybe removed as well. For this purpose, the control unit 4 determines byway of the temperature sensor 10 whether the temperature of the absorbercoating is high enough for regeneration of the absorber layer.

If the absorber coating temperature is below 500° C., the fuel injector12 injects fuel into the exhaust, which is catalytically burned with theoxygen present in the exhaust gas on the precious metal coating of theabsorber 1, raising its temperature. Alternatively and/or additionally,the metallic support for the absorber 1 can be electrically heated by aflow of current through the terminal 9. Still other arrangements forincreasing the absorber temperature, as for example inductive heating ofthe metallic support and/or a throttling of the exhaust are possible.

As soon as the absorber 1 is heated sufficiently, a rich mixture is setin the exhaust gas, i.e. by the fuel injector 12 and/or a rich mixtureas injected into the cylinder. Preferably a throttle 17 is adjusted inthe intake duct 16 of the combustion engine 3 by the control unit 4 sothat less air is supplied to the internal combustion engine 3. Thisdecreases the proportion of oxygen in the exhaust gas so that NO_(x) andSO_(x) are released from the absorption layer and are reduced.Termination of the regeneration may be time controlled or elsecontrolled by detecting the loss of exothermia effect of the reaction onthe exhaust temperature. In a further modification, the regeneration maytake place as described in U.S. Pat. No. 5,406,790 in which the exhaustgas flow is throttled ahead of the absorber and is directed to theabsorber through a by-pass.

To convert any hydrocarbons that may remain in the exhaust gas, an airinjector 13 located downstream of the absorber 1 is activated by thecontrol unit 4 during the fuel injection by the fuel injector 12. Inthis way, any hydrocarbons still remaining are oxidized to carbondioxide and water in the downstream catalyst 8.

If desired, the throttle 17 as well as the fuel injector 12 may beomitted, since the fuel injector 7 in the combustion chamber of theengine can enrich the exhaust gas sufficiently.

Since the output of the engine 3 may be reduced during regeneration ofthe absorbing layer, the system may be arranged so that, when the engineis operating at full power, regeneration may be suppressed at least fora certain length of time.

Although the invention has been described herein with reference tospecific embodiments, many modifications and variations therein willreadily occur to those skilled in the art. Accordingly, all suchvariations and modifications are included within the intended scope ofthe invention.

I claim:
 1. A method for removing at least one nitrogen oxide (NO_(x)from the exhaust gas of an internal combustion engine, comprising thesteps of: (a) operating an internal combustion engine to produce anexhaust gas flow containing oxygen; (b) passing exhaust gas containingoxygen over an absorber containing an absorbing layer on a surface of asupport member having a wall thickness≦140 microns; (c) storing theNO_(x) in the absorbing layer; (d) heating the absorbing layer to apredetermined temperature of at least 500° C. during the operation ofthe engine; (e) producing an exhaust gas which is poor in oxygen or anexhaust gas having a stoichiometric excess of a reducing agent; (f)desorbing the NO_(x) from the absorbing layer and reducing the NO_(x) inthe exhaust gas which is poor in oxygen has a stoichiometric excess ofreducing agent while the absorbing layer is a temperature equal to orabove the predetermined temperature; (g) again producing an exhaust gascontaining oxygen; (h) terminating heating of the absorbing layer to thepredetermined temperature; and (j) repeating steps (c) through (h).
 2. Amethod according to claim 1 wherein the step of heating the absorbinglayer is carried out by at least one step selected from the groupconsisting of: (a) injecting fuel into the exhaust gas and catalyticcombustion thereof, (b) varying the operating conditions of the internalcombustion engine, (c) electrical heating of the absorbing layer and (d)using a burner to heat the exhaust gas.
 3. A method according to claim 1wherein, before the step of heating the absorbing layer at least to apredetermined temperature during operation of the internal combustionengine, a step of determining whether a temperature value representingthe temperature of the absorbing layer is at or above the predeterminedtemperature is carried out and, if it is determined that the temperaturevalue representing the temperature of the absorbing layer is at or abovethe predetermined temperature, steps (d) and (b) are omitted.
 4. Amethod according to any one of claims 1-3 wherein the support member isa metal support member.
 5. A method according to any one of claims 1-3wherein at least one oxide of sulfur (SO_(x)) is also stored anddesorbed by the absorbent layer.
 6. A method according to any one ofclaims 1-3 wherein the desorption from the absorber layer is carried outat periodic intervals.
 7. A method according to any one of claims 1-3wherein the desorption from the absorbent layer is carried out dependingon the amount of gas stored in the absorbent layer.
 8. A methodaccording to any one of claims 1-3 wherein the absorbent layer containsgamma-aluminum oxide and at least one element in the group consisting ofalkali metals, alkaline-earth metals, rare earths and lanthanum.
 9. Amethod according to any one of claims 1-3 wherein the exhaust gas ispassed over the absorbent layer with turbulence.
 10. A method accordingto any one of claims 1-3 wherein the support member has a plurality ofparallel passages.
 11. A method according to claim 10 wherein theexhaust gas is passed over a plurality of support members containing thegas absorbing layer and having at least one step selected from the groupconsisting of: (a) different numbers of passages; (b) passages ofdifferent flow diameters; and (c) spacings between the support membersof at least 50 cm.
 12. A method according to claim 10 wherein thesupport member has a plurality of twisted or curved passages.